Loud explosions have been heard over the Afghan capital Kabul and electricity supplies have been cut.

US President George W Bush is due to make a statement 1250 (1650 GMT).

The explosions follow indications from Afghan opposition Northern Alliance sources that the United States was preparing to launch air strikes against the Taleban.

Alliance Foreign Minister Abdullah Abdullah said that unspecified American sources had told the anti-Taleban opposition to ground their aircraft and he warned residents of the capital Kabul to stay away from military bases.

We will never bow down to American forces and will fight till the last breath

Taleban Defence Ministry

The Taleban says it has sent 8,000 extra troops to Afghanistan's border with Uzbekistan, which has given American forces access to an air base for the US anti-terrorism campaign.

President Islam Karimov of Uzbekistan gave permission on Friday to visiting Secretary Rumsfeld for US cargo planes, helicopters and troops to use Uzbek territory - but only for search and rescue operations inside Afghanistan.

Within hours of Uzbekistan's announcement, elite US troops from the 10th Mountain Division began securing an airfield near the town of Termez near the Afghan border.

In other developments:

Taleban forces also send 2,000 troops to reinforce the town of Bagram, north of Kabul, which is one of the main areas of fighting with the opposition Northern Alliance

Pakistan and Iran agree to open their borders to receive refugees from Afghanistan in the event of strikes by American-led forces

Pakistan puts the leader of the pro-Taleban JUI party, Maulana Fazlur Rehman, under house arrest after he led a number of anti-US demonstrations

G7 finance ministers pledge tighter laws to track down funds used to further international terrorism

Donor countries pledge more than $600m for humanitarian needs in Afghanistan

A Taleban spokesman again says Osama Bin Laden could go on trial in Afghanistan and the offer is immediately rejected by Washington

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Afghanistan’s neighbours: Regional fears

Western leaders say military, diplomatic and humanitarian preparations have fallen into place for any attack on Osama Bin Laden and Afghanistan's ruling Taleban, nearly a month after the plane attacks on the US.

US President George W Bush has shown no interest in Taleban efforts to bargain over the fate of its guest Osama Bin Laden - Washington's prime suspect in the 11 September attacks on New York and Washington, which left more than 5,600 people dead or missing.

RAF forces are in Oman on exercise

"The Taleban has been given the opportunity to surrender all the terrorists in Afghanistan and to close down their camps and operations," Mr Bush said in his weekly radio address on Saturday. "Full warning has been given, and time is running out."

Mr Bush ignored questions about the Taleban on Sunday, as he attended a firefighters' memorial ceremony at the National Fire Academy in Emmitsburg near his Camp David retreat.

After the ceremony he quickly returned to the White House.

Some 35,000 US military personnel, 350 aircraft and three aircraft carrier battle groups are in the region.

Britain has made available more than 23,000 troops taking part in manoeuvres in the Gulf state of Oman.

Mr Bush also urged Congress to supplement $320m in immediate humanitarian aid with extra funds for the future reconstruction and development of Afghanistan.

Northern Alliance commanders say their forces have taken significant steps towards recapturing the strategic northern town of Mazar-i-Sharif.

"We will take Mazar in the next two to three days," a spokesman for opposition commander Atta Mohammad told the AFP news agency by telephone from northern Afghanistan.